FAQ

-How to Interface your DSC radio with a compatible GPS

Over the past few sailing seasons, I've had a number of members ask me about interfacing their DSC radios with a GPS. All VHF DSC Class D radios can receive positional data from a compatible GPS. If a distress situation arises, a DSC radio receiving GPS updates will relay that information as an integral part of the distress signal. Your position in a distress situation is most probably one of the most important components of any distress signal, and to not enable this function on your DSC radio is to not use, arguably, its most valuable feature. To supply your radio with co-ordinates of position from your GPS in most cases is easy. All you'll need are some basic tools and the owner's manuals for both the DSC radio and the GPS you wish to interface. Now that I have the time I've done a short write-up that should be of assistance to anyone who wants to tackle the task. It's impossible to cover all possibilities in detail in this brief discussion but it contains enough information to cover most installations. If anyone requires more information, or assistance, please feel free to contact me. If I'm in a position to offer either, I will certainly do so.

The first thing you must establish is what communications bus you will be using to interface the devices. It is important to understand that both the GPS and the radio you wish to interface must use the same interface protocol or the GPS will not be able to "talk" to the radio. Or, for that matter, the radio will not be able to "listen" to the GPS. Here the owner's manuals will be your guide. Owner's manuals have different formats but the most likely place you will find the information you are looking for is under Interfacing, where the protocols are usually listed. The interface protocols presently in use are listed below and you must ascertain from the owner's manuals that one of these protocols is common to both devices:

NMEA 0183 - This specification for an ASCII serial data bus protocol has been around since 1983 and is by far the most common bus used for communications between marine electronic equipment. It is similar, but not identical to, the RS232 bus on your computer. Even the majority of equipment manufactured after 2000, when NMEA 0183 was superseded by NMEA 2000, still provide the NMEA 0183 interface. NMEA 0183 comes in different versions. The latest is Version 4.00, which replaces Version 3.01, published in January of 2002, and is backwards compatible all the way to Version 2.00.

NMEA 2000 - The NMEA 2000 is a digital bus and has a significantly higher data rate (250,000 baud vs. 4,800 baud for NMEA 0183). It uses a compact serial binary message format. Furuno is the only manufacturer that I'm aware of that uses NMEA 2000 on both GPSs and DSC radios. (This does not mean to say that other manufacturers don't do this.) Interfacing NMEA 2000 with NMEA 0183 using a bridge or gateway network is possible, but is expensive and beyond the scope of these simple how-to instructions. I doubt anyone will have to go through this process, but if anyone who wants to know more about this he/she can contact me.

Proprietary protocols - Several manufacturers have developed their own protocols so their equipment can communicate with each other without having to use any of the NMEA standards. Although they make their proprietary protocols available for use, to ensure their equipment can still also be interfaced with equipment from other manufacturers, they invariably provide the NMEA 0183 interface as well. The manufactures who have their own proprietary protocols include:

Raymarine - SeaTalk

Furuno - NavNet

Garmin - Garmin Marine

Stowe - Dataline 2000

Brunswick - SmartCraft

Unless your GPS and radio are manufactured by the same manufacturer, who makes a proprietary protocol available, and you choose to use that protocol, it is almost certain that you will using the NMEA 0183 bus. See the abbreviated table below.

GPS

DSC Radio

Communications Bus

Raymarine

Raymarine

Seatalk

Furuno

Furuno

NMEA 2000

All other manufacturers*

All other manufacturers*

NMEA 0183

*Study the owner's manuals for your equipment carefully because there are oddballs out there. There may also be other manufacturers who use NMEA 2000, and also the odd manufacturer that does not provide a NMEA 0183 interface. (The Garmin 18 GPS, for example, uses a USB port that is not compatible with NMEA 0183.) Remember the same bus protocol must be available on both devices or they will not interface.

Once you have established, after carefully consulting the owner's manuals, that a mutually compatible communications protocol is available on both the GPS and the DSC radio you wish to interface, you can move on to the next step.

Wiring

Proprietary Bus e.g. Seatalk

All you need to make the connection between two Raymarine devices is to connect a Seatalk cable between them. The Seatalk connectors are clearly identified. The cable just plugs into both devices and you are ready to roll. Cables for use on other proprietary busses are specified in the owner's manuals and should be available for purchase at any chandlery.

NMEA 2000

The system is based on a network topology, with T connectors, drop lines and has to be properly terminated in 120 ohms at each end, resulting in a 60 ohm impedance. It does not allow daisy-chaining of equipment so the connection of NMEA 2000 devices must be carefully planned ahead. All cables, connectors, terminators etc. should be NMEA 2000 approved and the wiring here is installation dependent. It is impossible to come up with a brief how-to description to cover all possibilities. Furuno for example has wedded NMEA 2000 with the Ethernet, meaning you can access packaging data via a laptop. Again I'll be pleased to assist anyone with more information if requested.

NMEA 0183

Identifying these connections is easy. They are nearly always contained in a pigtail coming out of the power plug and are identified by colour in the owner's manual. Remember to use shielded twisted pairs to extend existing wires. Also the shield must be grounded at one end only to prevent ground loops. To select the proper wires in your existing configuration you have to think in terms of input and output. NMEA 0183 devices have data input ports as well as data output ports. In other words your DSC radio will have data input ports as well as data output ports. So will your GPS. Just remember you want to feed data from the GPS to your radio, so you will require Data Out from the GPS and Data In to the radio. Note that the jargon varies between manufacturers. Data Out is also sometimes identified as NMEA 0183 OUT +, and Data In as NMEA 0183 IN +.

Two examples of different GPSs, and one example of a radio, are shown below. This should suffice because all NMEA 1083 interfaces will be similar no matter the make or model of your equipment. Note that for the Garmin GPSmap76 the black or common wire also serves as the NMEA 0183 OUT- connection. This is referred to as a 3 wire NMEA connection because the black also serves as NMEA 0183 IN- if the input port is used.

GPSmap76

Red: Power

Black: Common (NMEA 0183 OUT -)

Brown: Data Out (NMEA 0183 OUT +)

White: Data In (NMEA 0183 IN +) (not needed)

The Raymarine Raynav 300 uses the classic 4 wire connection where the input and output commons are separate.

Raynav 300 GPS Plotter

White: Data in (NMEA 0183 IN +)(not needed)

Green: Data In (NMEA 0183 IN -)(not needed)

Yellow: Data Out (NMEA 0183 OUT +)

Brown: Data Out (NMEA 0183 OUT -)

The Uniden UM 625c VHF DSC radio also uses the 4 wire connection.

Uniden UM 625c Radio

White: Data in (NMEA 0183 IN +)

Yellow: Data In (NMEA 0183 IN -)

Green: Data Out (NMEA 0183 OUT +)(not needed)

Orange: Data Out (NMEA 0183 OUT -) (not needed)

To connect either of these GPSs to the radio examine the table below.

GPSmap76

Raynav 300

Uniden UM 625c

Brown (Data out +)

White (Data in +)

Black (Data out -)

White (Data in -)

Brown (Data out +)

White (Data in +)

Brown (Data out -)

White (Data in -)

Setting up the NMEA 0183 interface on the GPS

Once you have made the connections between the GPS and DSC radio, and you switch both on, the odds are your radio will not be receiving updates. It is easy to recognise if updates are taking place because the display on the radio will show a flashing satellite or other easily recognisable symbol to indicate the radio is receiving positional data. If the radio is not receiving updates - which is bound to be the case - it is because the GPS has not been set up to output the NMEA 0183 protocol. It is easy to set up on most GPSs. Menus come in different flavours on different GPSs. The interface setup menu is usually easily accessible from the system setup or top level menu. From there the interface menu is selected. As a general guide here is how it is accessed from the Garmin GPSmap76.

From main menu select SETUP.

The following options are shown horizontally at the top of the screen:

Scroll down and select NMEA (the default Baud rate of 4800 is then displayed below it).

Your GPS is now correctly setup to output NMEA 0183 data, and if the GPS has established a fix then it should be updating the DSC radio with this data.

In case you're wondering about the GARMIN, GARMIN DGPS interface selection options, these are the Garmin Marine proprietary protocols mentioned earlier. The DGPS one can be used if a ground station signal is available to allow for Differential GPS operation.

NOTE: If there is any uncertainty in your mind if the radio is indeed being updated, there is a quick and dirty method of finding out. Let the GPS talk to the radio for about a minute or so. Now switch off the GPS. Within a few seconds the radio should start to complain. Mine actually emits a high-pitched goose-flesh-inducing alarm then scrolls a message across the display: Lost satellite information.